There are no true viruses for Mac OS X (not to be confused with the more general category of "Malware", which can consist of trojans, worms, adware and spyware). There are some types of malware that effects OS X, but it is very rare and none of it self-replicates.

It is theoretically possible that you could have an infected file, that while being harmless to your Mac, could harm a Windows machine if you transfer it to someone else.

Do a full scan with ClamXAV. If it doesn't find anything, then your friend probably didn't get it from you.

Liquid and computers don't mix. It might seem simple, but we see an incredible amount of people post here about spills. Keep drinks and other liquids away from your expensive electronics!